1. Field of the Invention
The invention concerns a self-commutating inverter with controlled main valves in a center-tap circuit, with which a commutation capacitor, a series choke and a recovery valve each are associated, and with a transformer for connecting a variable load.
2. Description of the Prior Art
Such an inverter (see, for instance, the Siemens text book Thyristor Handbuch (Thyristor Handbook) by A. Hoffman and K. Stocker, 1965, Berlin and Erlangen, pages 243 to 250, particularly FIG. 165 on page 248) can be supplied, for instance, from a battery or from a rectifier with controlled or uncontrolled valves. It delivers at the secondary winding of its transformer a rectangular or trapezoidal a-c voltage the amplitude of which is determined by the magnitude of the input d-c voltage and the frequency of which is determined by the firing frequency of the main valves. In such an inverter, the recovery valves may be connected on the cathode side to the two ends of the primary winding of the transformer. In deviation therefrom, it has become customary to provide the primary winding with taps in the vicinity of its two ends and to connect the recovery valves there. In order to prevent the commutation capacitor from partially discharging prematurely by way of the primary winding of the transformer, the inverter is further supplemented by two series diodes which are connected in series with each of the main valves. Arranging these series diodes is absolutely necessary if the load is subject to variations during the operation of the inverter. Also in the event of variations of the input d-c voltage, the series diodes prevent the commutation capacitor from discharging prematurely, so that also in this case reliable commutation from one main valve to the other is assured. Such an inverter can also be supplemented to form a bridge circuit (see the literature reference mentioned, "Thyristor Handbuch", particularly FIG. 167 on page 250).
In an inverter with a center-tap circuit, there must flow through a series choke, in order to avoid voltage dips, i.e., deviations from the desired rectangular or trapezoidal shape of the output voltage, a circulating current which, considering the transformation ratio of the tansformer, corresponds to at least the magnitude of the maximum load current to be expected. This circulating current is determined not only by the voltage at the transformer taps that may be provided, but additionally also by the line resistances and the path resistances of the valves in the circuit in question.
Assuming that the tap voltages at the transformer, which cause a reversal of the magnetization of the series choke, and their effective times are larger than the voltage and the charge reversal time of the commutation capacitor, no continuously flowing circulating current will develop. The ensuing circulating current will have gaps.
If, however, the tap voltages at the transformer are made smaller, a non-gapping circulating current will always develop. The latter produces at the line resistances and at the path resistances of the valves an additional counter-voltage for the series choke, so that an equilibrium state between forward magnetization and reverse magnetization is brought about. If the inverter is idling, i.e., if the inverter, for instance, has no load connected to it, the continuously flowing circulating current will generate relatively high losses. In general, these idling losses are tolerated, as an inverter with continuously flowing circulating current has the advantage that a suddenly added load or a load jump does not cause a break in the output voltage. Particularly in an inverter of high operational power rating, the trend is, however, to keep these idling losses caused by the circulating current to a minimum. In switching systems, in which the idle condition of the inverter corresponds to a ready-reserve position, e.g., in ready-standby installations, the operating costs can be reduced by a reduction of the idling losses.
It is an object of the invention to reduce the losses of the inverter mentioned at the outset, which is basically designed for operationally varying loads, to increase the efficiency thereby and to lower the cost of operation.